The invention is in the field of analog to digital (A/D) converters and is particularly directed to an A/D converter which is suitable for fabrication according to integrated circuit techniques.
There are various types of A/D converters known. These include the single ramp type, the double ramp type, voltage to frequency converters (VCO's), the comparison type which includes a D/A converter and compares the output to the input, etc. All present A/D converters have their respective problems such as inaccuracy, high cost, etc. For example, accuracy is a problem with A/D converters using integrators because of the difficulty in achieving linear ramp voltages and maintaining fixed voltage crossing points. Also VCO's are basically non-linear. Those A/D converters with D/A feedback may be accurate but are relatively expensive.
Substantially all A/D converters have the problem of error due to drift or variation in components and control signals caused by temperature, environment and age variations. Several schemes have been devised to compensate for such errors in various types of A/D converters. Many of these techniques are quite complicated.
An accurate A/D converter that can be integrated into one monolithic chip has been reported by Chellen et al. in a paper given at the IEEE International Solid-State Circuits Conference in Philadelphia in February, 1973. The report appears on page 22 of the proceedings of the conference. The A/D reported utilizes a voltage controlled oscillator responsive to an input and a reference voltage. The VCO output and a submultiple of the system clock are applied to an edge responsive flip-flop whose output in turn controls the application of V.sub.REF to the VCO input. The VCO with feedback is used to alter the duty cycle of the flip-flop output in such a way that the duty cycle is a linear function of the input analog signal, i.e., the up-down time ratio is dependent upon the ratio of the input and reference voltages and in that sense the A/D is self-compensating. Accuracy of the system, however, depends upon the ratio of the input resistors to the VCO and on the linearity of the VCO. As is known a VCO is not sufficiently linear for high accuracy, but the authors claim the self-compensating feature compensates to a sufficient degree for the non-linearity of the VCO.
Willard et al., U.S. Pat. No. 3,530,458, teaches an A/D converter with feedback means to compensate for drift in the gain and zero characteristics of the conversion system. However, an integral part of the compensation system is a digital computer, and relays and motor operated rheostats are necessary to alter the input bias resistance (zero characteristic compensation) and to automatically switch tapped resistors (gain characteristic compensation). These elements result in high cost and are not conducive to fabricating the A/D into a simple integrated circuit structure. Furthermore, the system utilizes a voltage to frequency converter which is inherently non-linear. The patent states that the system is only suitable for low frequency input analog voltages, e.g., below 5 Hz.